1. Field of the Invention
The instant invention relates to the production of microspheres, and more particularly to the production of gelled microspheres which comprise a compound that includes a non-metal element and a metal element. In its highly preferred aspects, the invention relates more specifically to the production of microspheres of a nuclear fuel material such as oxides, carbides, nitrides of uranium and/or plutonium.
2. Description of the Prior Art
Processes for the production of microspheres consisting of at least one compound that includes a metal element and a non-metal element such as oxygen, carbon or nitrogen, by internal gelling of droplets of a mixed feed solution which contains an ammonia doner such as hexamethylene tetramine in a concentrated metal salt solution are known.
Generally, one starts with a so-called mixed feed solution which is customarily prepared by admixture of an ammonia donor such as hexamethylene tetramine in a saturated metal nitrate solution. The mixture remains fluid as long as it is maintained at a low temperature, such as, for example, 5.degree. C. But the mixture gels when it is heated.
It has been known previously to divide the cool mixed feed solution into droplets of as uniform a size as possible and to then introduce the droplets into a bath of hot hydrophobic liquid, such as, for example, silicon oil, so that the droplets will rigidify very quickly by gelling as a consequence of a great pH increase due to the formation of ammonia from the hexamethylene tetramine. Any hydrophobic liquid which adheres to the rigidified gelled microspheres may be removed with a chlorinated hydrocarbon. Subsequently, the gelled microspheres are washed in an ammoniac solution, preferably in a countercurrent process, in order to leach out soluble compounds, such as ammonium nitrate, hexamethylene tetramine and urea.
Such internal gelling processes have frequently, although not exclusively, been used for the production of ceramic nuclear fuels in the form of microspheres. For example, for production of microspheres of a uranium-plutonium nuclear fuel, the mixed feed solution may consist of a saturated aqueous solution of uranyl nitrate and plutonium nitrate which includes additives to facilitate the solidification reaction such as hexamethylene tetramine and urea. The solidified gelled-microspheres are then sintered after washing and possibly after pre-conditioning by heat treatment in a gaseous atmosphere of pre-determined composition at an elevated temperature. However, such wet chemical production process for nuclear fuel has considerable advantages, as compared to production from powders, since no dust contaminated with radioactive material is obtained. But the wet chemical production of nuclear fuel microspheres must be accomplished in an environment which provides radiation protection, and thus, it turns out that safety and precautionary measures for processes which involve the gelling of the drops in hot hydrophobic liquid are expensive and difficult to achieve.